1. Field
The present invention relates to a method for repairing a jet pump and more particularly to a method for repairing the slip joint between an inlet mixer and a diffuser of a jet pump with particular benefit to jet pumps employed in boiling water reactors.
2. Related Art
As can be appreciated from FIG. 1, in conventional boiling water reactors 10, jet pump assemblies 18 are located in the reactor vessel's annulus region 12, between the core shroud 14 and a wall of the reactor vessel 16. The primary function of the jet pump is to pump coolant below the reactor core where the coolant then flows up through the fuel assemblies to extract heat from the fuel assemblies. The jet pumps are also relied upon to maintain two-thirds of the water level height in the reactor core during postulated accident conditions. For these reasons, the jet pump assemblies 18 are considered safety related components.
In a typical arrangement, twenty jet pumps are paired in ten assemblies 18, as illustrated in FIG. 2. Each jet pump assembly 18 is driven by flow from a common riser pipe 22. Jet pump flow is then directed to the lower plenum region 24 below the core supported within the shroud 14. Jet pump flow is roughly one-third driven by the reactor coolant system pump flow through the riser pipe 22, and the last two-thirds of jet pump flow is due to venturi action of the jet pump suction inlet that pulls in fluid from the annulus region 12. The jet pump assemblies 18 are circumferentially spaced around the shroud 14, supported on a shroud support ledge 20 near the bottom of the shroud.
Industry operating experience has encountered numerous instances of damage or accelerated wear to critical components of the jet pump assemblies 18 which has affected a large population of boiling water reactors both in the United States and globally. The most common jet pump components to experience damage are the main wedge and rod 26, restrainer bracket pad in the restrainer bracket 28, riser pipe 22 welds and riser brace 30, all of which components can be observed in FIG. 3, which shows an enlarged perspective view of a jet pump assembly 18. These common types of damage are all in critical supporting structures or features of the jet pump assemblies. Damage and wear are largely attributable to flow induced vibration due to normal operation. In addition to flow induced vibrations, many plants experience excessive leakage in the slip joint region 32, which can exacerbate the vibration experienced in the jet pump assemblies 18. This leakage can greatly accelerate the degradation and damage done to the jet pumps.
Plants that have shown signs of accelerated wear and component movement, both indicators of flow induced vibration issues, have historically attempted to address the problem by adding additional hardware to help support the jet pump components and reinforce the components against flow induced vibration. These solutions have generally been in the area of the restrainer bracket 28 and include larger main wedges 26, supplemental auxiliary wedges, and slip joint clamps. These solutions do not address the root cause of flow induced vibrations, have been ineffective for some plants, and their effectiveness overall is questionable.
In addition to these typical solutions, a few boiling water reactor plants have added labyrinth seals to the inlet mixer 34 original equipment manufacturer design. These labyrinth seals are intended to reduce bypass flow in the slip joint region 32 of the jet pumps; however, it appears that under certain operating conditions in some plants these seals have been ineffective and the seal geometry has been damaged on the inlet mixer's outer diameter and has caused damage to the inner diameter of the collar 38 on the diffuser 36.
In the slip joint region 32, the inlet mixer 34 is unsupported or floats, allowing the mixer to thermally expand along its length during plant startup and shutdown. The inlet mixer's bottom section fits into the collar 38 of the diffuser assembly 36, forming a slip joint. The inlet mixer 34 is laterally supported by a three-point contact at the restrainer bracket 28. This three-point contact is maintained with a sliding (main) wedge 26 and two set screws that are tack welded in place. The main wedge 26 is held in place by gravity, theoretically resulting in three-point contact. The very upper portion of the inlet mixers are supported by a pre-tensioned beam bolt assembly 40 that presses down on the inlet mixer 34 where it is seated in the transition seat 42.
Because of how the inlet mixers 34 are supported, small lateral loads on the bottom of the inlet mixer (within the slip joint 32) can create large reaction moments at the restrainer bracket 28. As previously mentioned, the main wedge 26 is held in place by its weight, typically about eight pounds, which can be overcome and lifted by small lateral forces. Since the inlet mixer 34 weighs significantly more than the wedges 26, its mass can easily overcome the holddown force of the main wedge 26 with small lateral displacements at the outlet of the inlet mixer 34 within the diffuser collar 38. Once the wedge is temporarily displaced, three-point contact is lost, and, in severe cases, the bottom of the inlet mixer may hammer against the inside of the diffuser collar 38. This hammering of the inlet mixer 34 and diffuser 36 can also be excited at particular frequencies of vibrations, potentially caused by drive flow or bypass flow in the slip joint 32.
Thus, a new solution to flow induced vibrations is desired that will address the root cause of the vibrations.
Furthermore, a solution to the flow induced vibration wear is desired that will minimize such wear and require little or no disassembly of the jet pump assembly 18.
Further, such a repair is desired that can be performed remotely, under water.